Coronal hole boundaries evolution at
small scales

II. XRT view. Can small-scale outflows at
CHBs be a source of the slow solar wind?

S. Subramanian - M. S.
Madjarska - J. G. Doyle

Armagh Observatory, College Hill, Armagh
BT61 9DG, N. Ireland

Received 8 November 2009 / Accepted 12
April 2010

AbstractAims. We aim to further explore the small-scale
evolution of coronal hole boundaries using X-ray high-resolution and
high-cadence images. We intend to determine the fine structure and
dynamics of the events causing changes of coronal hole boundaries and
to explore the possibility that these events are the source of the slow
solar wind. Methods. We developed an automated procedure for the
identification of transient brightenings in images from the X-ray
telescope on-board Hinode taken with an Al Poly filter in the
equatorial coronal holes, polar coronal holes, and the quiet Sun with
and without transient coronal holes. Results. We found that in comparison to the quiet
Sun, the boundaries of coronal holes are abundant with brightening
events including areas inside the coronal holes where closed magnetic
field structures are present. The visual analysis of these brightenings
revealed that around 70% of them in equatorial, polar and transient
coronal holes and their boundaries show expanding loop structures
and/or collimated outflows. In the quiet Sun only 30% of the
brightenings show flows with most of them appearing to be contained in
the solar corona by closed magnetic field lines. This strongly suggests
that magnetic reconnection of co-spatial open and closed magnetic field
lines creates the necessary conditions for plasma outflows to large
distances. The ejected plasma always originates from pre-existing or
newly emerging (at X-ray temperatures) bright points. Conclusions. The present study confirms our findings
that the evolution of loop structures known as coronal bright points is
associated with the small-scale changes of coronal hole boundaries. The
loop structures show an expansion and eruption with the trapped plasma
consequently escaping along the ``quasi'' open magnetic field lines.
These ejections appear to be triggered by magnetic reconnection, e.g.
the so-called interchange reconnection between the closed magnetic
field lines (BPs) and the open magnetic field lines of the coronal
holes. We suggest that these plasma outflows are possibly one of the
sources of the slow solar wind.

1
Introduction

Coronal holes (CHs) are regions of predominantly unipolar coronal
magnetic fields with a significant component of the magnetic field open
into the heliosphere. They are visible in spectral lines emitting at
coronal temperatures as dark areas when compared to the quiet Sun,
while in the chromospheric He I 10830 Å
line they appear bright. For detailed introduction on coronal holes see
Madjarska &
Wiegelmann (2009, hereafter Paper I). CHs are
identified as the source of the fast solar wind with velocities of up
to 800 km s-1
(Krieger et al.
1973). In contrast, the slow wind has velocities around
400 km s-1 and is more dense,
and variable in nature when compared to the fast solar wind. von Steiger (1996)
found from Ulysses satellite data that the elemental composition of the
fast wind is similar to the elemental composition of the photosphere.
The slow solar wind is enriched with low first ionization potential
(FIP) elements by a factor of 3-5 greater than in the
photosphere (with respect to hydrogen) while higher FIP elements were
found at solar surface abundances. The FIP effect describes the element
abundance anomalies (the enhancement of elements with low FIP such as
Fe, Mg and Si over those with high FIP like Ne and Ar) in the upper
solar atmosphere and solar wind, and can give a clue on the origin of
both the fast and the slow solar winds. von Steiger (1996)
concluded that the fast and slow solar winds not only differ in their
kinetics but also in their composition of elements.

Woo
et al. (2004) suggested that the release of trapped
plasma in closed loop structures by magnetic reconnection could play a
significant role in the solar wind flow. Such reconnection between the
open and closed magnetic field lines presumably happens continuously at
coronal hole boundaries. Wang
et al. (1998) investigated the ejection of plasma
blobs from the streamer belt linked to the slow wind and concluded that
magnetic reconnection between the distended streamer loops and the open
magnetic field lines might be behind the plasma ejection. They also
suggested that this ejection cannot account for all the slow solar wind
and a major component should, therefore, originate outside the helmet
streamers, i.e. from inside the coronal holes. Madjarska et al. (2004)
found non-Gaussian profiles along the boundaries of an equatorial
extension of a polar CH in the mid- and high-transition region lines
N IV 765 Å and
Ne VIII 770 Å,
respectively, recorded with the Solar Measurement of Emitted Radiation
(SUMER) spectrometer on-board the Solar and Heliospheric Observatory
(SoHO). The authors suggested that these profiles are the signature of
magnetic reconnection occurring between the closed magnetic field lines
of the quiet Sun and the open of the coronal hole. Similar activity was
reported by Doyle
et al. (2006) along the boundary of a polar CH.

In Paper I we demonstrated that although isolated
equatorial CH and equatorial extension of polar CH maintain their
general shape during several solar rotations, a closer look at their
day-by-day and even hour-by-hour evolution demonstrates significant
dynamics. We showed that small-scale loops which are abundant along
coronal hole boundaries contribute to the small-scale evolution of
coronal holes. We suggested that these dynamics are triggered by
continuous magnetic reconnection already proposed by
Madjarska et al.
(2004). The next step of our research was to analyse images
taken with the X-ray Telescope (XRT) on-board Hinode.

Seen in XRT images, CHs are highly structured and dynamic at
small scales. High cadence XRT data reveal in great detail the fine
structure of coronal bright points (BPs) and X-ray jets associated with
them. X-ray jets are collimated transient ejection of coronal plasma,
first reported with the Solar X-ray telescope (SXT) onboard Yohkoh (Shibata et al. 1992).
They are believed to result from magnetic reconnection (Shibata et al. 1994)
and represent plasma outflows from the reconnection site. Recently, Moreno-Insertis et al.
(2008) presented three-dimensional simulations of flux
emergence in CH combined with spectroscopic and imager observations
from XRT and EIS/Hinode of an X-ray jet. The authors report that a jet
resulting from magnetic reconnection is expelled upward along the open
reconnected field lines with values of temperature, density, and
velocity in agreement with the XRT and EIS
observations. Shimojo
et al. (1996) reported 100 jets over 6 months in SXT
images from the Yohkoh while Savcheva
et al. (2007) upgraded this number to an average of
60 jet events per day in polar coronal holes. The authors concluded
that jets preferably occur inside polar coronal holes (PCH).

We should note that although TRACE images which have higher
spatial resolution were used in Paper I, the detailed
structure of the dynamic changes along CH boundaries was hard to
distinguish. A reason for that is the effect of stray light in the
TRACE extreme-ultraviolet (EUV) telescope reported recently by DeForest et al. (2009).
The authors found that 43% of the light which enters TRACE through the
Fe IX/X 171 Å filter is
scattered
either through diffraction off the entrance filter grid or through
other non-specific effects. This creates a haze effect and especially
effects the visibility of small-scale bright structures.

Other transient structures seen in coronal holes are the
so-called plumes observed off-limb above the North and South polar
coronal holes. They were first observed in white light as ray like
structures (Saito 1965).
They are also observed at EUV and soft X-ray temperature (Ahmad & Webb 1978)
as coronal outflow structures similar to coronal jets, but hazy in
nature with no sharp boundaries unlike jets. They represent denser and
cooler outflows with respect to the surrounding media and are observed
to extend from coronal BPs. They can extend up to
from the solar disk center in a plane image (DeForest et al. 2001)
and are observed to be in a steady state for at least 24 h (Deforest et al. 1997).
The X-ray jets have been identified as precursors for the plume
formation (Raouafi
et al. 2008). Recently, Wang & Muglach (2008)
identified coronal plumes inside equatorial coronal holes. They found
that the plumes are analogous to polar coronal plumes. On the disk they
are seen as a diffuse structure with a bright core and associated with
EUV BPs.

The present study is a continuation of Paper I and
presents results from the analysis of high-cadence/high-resolution
images of coronal holes (equatorial, polar and transient) and quiet Sun
from XRT/Hinode. We aim to establish which type of event generates the
non-Gaussian profiles registered at CH boundaries by Madjarska et al. (2004)
and how they are related to the small-scale BPs evolution along coronal
hole boundaries as reported in Paper I. In Sect. 2 we describe the data
used for our study. Section 3
outlines an automatic brightening identification procedure. In
Sect. 4,
we give the obtained results and draw some conclusions on the outcome
of our study. Finally, in Sect. 5 we discuss
the implication of our result to the understanding of the nature of
coronal hole boundaries evolution at small scale and the possible
contribution of these events to the formation of the slow solar wind.

2 Data, reduction and preparation

We used images from the X-ray Telescope (Golub et al. 2007)
on-board Hinode taken during a dedicated observing run of an isolated
equatorial coronal hole (ECH), a Southern polar coronal hole and quiet
Sun regions. The ECH was tracked from the West to the East limb from 8
to 10 h per day for 4 days. The Southern polar CH was observed
for one day while the quiet Sun regions over 2 days. All data
were taken with an Al Poly filter which has a well pronounced
temperature response at
K.
XRT images have an angular pixel size of
1
1
at full resolution. They were
recorded with 16 s and
23 s exposure time and a cadence of about 40 s. We
also used randomly selected quiet Sun data with transient coronal holes
(TCHs) and a Northern polar coronal hole observation. Further details
on the data can be found in Table 1.

The data were reduced using the standard procedures, which
include flat-field subtraction, dark current removal, despiking,
normalisation to data number per second to account for the variations
in exposure time, satellite jitter and orbital variation corrections.
The images were then de-rotated to a reference time to compensate for
the solar rotation. A common field-of-view (FOV) was selected from all
the images for each day. We then prepared an array with dimensions (nx,
ny, nf), where nx
is the number of Solar_X pixels, ny is the number
of Solar_Y pixels and nf is the number of images.
Each image was binned to pixels2
in order to improve the signal-to-noise ratio and reduce the data
points (and subsequently the computational time). The binned images
were used to produce light curves of nf points for each pixel. These
light curves were the input for an identification procedure which will
be discussed in the next section.

Figure 1:

Equatorial coronal hole ( top left), polar CH (
top right), quiet Sun ( bottom left) and
quiet Sun with TCHs ( bottom right) with the
positions of all the corresponding identified brightening pixels
over-plotted. The CH boundaries are outlined with a black line. The
over-drawn rectangles correspond to the field-of-views shown in
Figs. 5-7. Time sequences can
be found in movies online.

3 Brightening identification procedure

We developed an automatic identification procedure to distinguish
small-scale intensity enhancements in XRT images. While the visual
identification of large events such as jets from bright points give
good results, it is difficult to identify and track small-scale events,
especially on the quiet Sun high background emission or over
pre-existing bright coronal loop structures (e.g. bright
points, active regions etc.). We eliminated all light curves which show
no activity or minimum activity comparable to the noise level.

The first step of the identification procedure was to define
the background emission for each light curve. The light curves were
smoothed over a window of 5 frames to remove the spikiness in
the background. Due to a difference in the background emission between
the quiet Sun and the CHs, it was necessary to set two different
thresholds for further analysis. The thresholds we used were
1.8 times the mean emission value for the CHs and
1.3 times the mean emissivity for the QS. The comparatively
higher threshold set for the CH light curves helped to eliminate the
high fluctuations of the low emission background. Light curves with a
maximum value less than these thresholds were neglected. Any point in
the light curve was considered as a peak if its value was greater than
the threshold and also greater than the average of the
two preceding points, and the average of two successive points. All the
values below the threshold were considered as local minima. Each
identified peak was traced back on either side to identify the minimum
from the local minima. The value of all the points between the two
identified minima for each peak were set to zero in the light curve and
thereby from the average over the rest of the light curve (
), we computed the standard
deviation (SD). The new background (BG) was obtained as
.

The next step was the actual identification of intensity
enhancements. A new threshold of 2 BG for CHs and
1.3 BG
for QS was set using the above calculated background. Intensity
increases above these thresholds with corresponding minima less than BG
and duration less than 45 min were identified. A pixel
brightening was considered only if all the above mentioned conditions
were satisfied. The threshold was calculated with a trial and error
method.

The peaks having a duration of more than 45 min were
examined separately. The closest local minimum on either side of the
peak were traced back. If the difference between the peak and the
minimum were greater than the BG for the CHs and
for the QS with the duration less than 45 min, then they were
considered. Also the peaks which have one minimum that was either in
the beginning or at the end of the light-curve were evaluated with the
same criteria, in order not to miss any real event. Any intensity
enhancements in the coronal holes, the quiet Sun or over pre-existing
bright loop structures which satisfy the above criteria could be
identified by our procedure.

4 Results and discussion

As it has been described in Sect. 2, we made a selection of
data which comprised observations of different features on the Sun:
equatorial and polar coronal holes as well as quiet Sun regions with
and without transient coronal holes. In Figs. 1-4 we display examples
of an X-ray image from each different region. Our intention was to find
out whether the changes we have seen so far along CHBs (Madjarska et al. 2004;
and Paper I) are unique for CH regions, i.e. regions of open
magnetic field lines. These data also permit to resolve the fine
structure of individual features and follow their dynamics at high
cadence. To each dataset we applied the identification procedure
described above. This procedure provided us with the following
information: (i) light curves which contain one or more radiance
enhancement identified as brightenings following the criteria given in
Sect. 3; (ii) the start and end time of each
radiance enhancement; (iii) the brightening positions in pixel numbers.
As we produced light curves by binning over 4 4
pixels2, imprints of brightening events with
spatial scales larger than 4 4 pixels2
were observed in more than one light curve. This made visual grouping
of identified bright pixels essential to distinguish each event.
Grouping of the features into individual events was done by playing the
image sequence of each dataset with the identified brightenings
over-plotted at corresponding times (see the online movies). Clusters
of bright pixels identified next to each other with similar lightcurves
were grouped into events. The events showing plasma outflows (i.e.
plasma moving along quasi-straight trajectories) were classified as
jets, while events exhibiting plasma blobs moving along curved
trajectories or just brightening increase in a group of pixels were
classified as unresolved brightenings events. The so-called space-time
plot was also used to investigate the plasma motion in the form of a
jet and to determine their proper motion. A space-time plot was
produced by averaging over a slice of 3 pixels wide and 100 pixels long
from each image, cut along the jet, i.e. in the direction of plasma
propagation and then plotting that in time (Shimojo et al. 2007;
and Subramianian, Ph.D. thesis 2010). We were able to group more than
of the identified bright pixels. The ungrouped pixels (5%) comprise
bright pixels identified at the edges of images and above bad pixels.
The pixels identified in the beginning of each dataset which could not
be classified due to the lack of coverage of the whole event and the
pixels identified with a time lapse over their lifetimes were also
rejected from counting.

The visual grouping of identified bright pixels into events
can be found in Table 2.
We defined a coronal hole boundary region (CHBR) as the region 15
on both sides of the contour
line defining the CH boundary.
Additionally, animated image sequences with over-plotted identified
brightenings at corresponding times are available online (cf.
Fig. 1).

The first and the most important result of this study is
easily noticeable from Figs. 1-4 the boundaries of
coronal holes are abundant with brightening events which appear much
larger than the same phenomena in the quiet-Sun region. We separated
the events visually into two groups, events with plasma outflows or
jet-like events and events without outflows or simple brightenings. The
equatorial coronal hole data, observed near the disk center, show twice
as many jet-like and simple brightening events in the CHBRs (as defined
above) as compared to the CH regions. In contrast, polar
coronal hole data and ECH close to the West limb (2007 November 16)
show a higher number of events inside the CH as well as in the CHBRs
suggesting that this can be due to the line-of-sight effect. However,
further investigation is needed on a larger number of datasets.

Figure 2:

Polar coronal hole observed by XRT on 2007 September 20 with the
positions of all the identified brightening pixels over-plotted.

Equatorial coronal hole observed by XRT on 2007 November 9, 14 and 16
with the positions of all the identified brightening pixels
over-plotted. The CH boundaries are over-plotted with a black solid
line.

Table 2:
Number of events over 24 h per 100
100 arcsec2 identified inside the coronal holes
(CH), in coronal hole boundary regions (CHBR) and in the quiet Sun (QS).

If we assume that the magnetic reconnection responsible for
the occurrence of jet-like events takes place predominantly between
closed and open magnetic field lines, then the number of reconnection
events producing outflows will be always higher in the CH boundary
region since open and closed magnetic field lines are continuously
pushed together by different processes such as convection, differential
rotation, meridian motions etc. Inside coronal holes, where the number
of bipolar systems and the corresponding closed loop structures are
limited, the number of jet-like events will therefore be lower. For the
transient coronal hole regions, the separation of the coronal hole
boundaries region (30
wide) from the coronal holes,
for estimating the number of events in
each region, is more difficult due to the very small size of these
coronal holes. Therefore, here we consider that these CHs represent
entirely a boundary region. The number of events found in these TCHs is
several times larger than in the quiet Sun (both with and without
outflows).

The plasma ejected during the outflow events always originates
from pre-existing or newly emerging (at X-ray temperatures) bright
points
both inside CHB regions and CHs. They typically start with a
brightening in just a few XRT pixels (4-6) somewhere in a
pre-existing BP which we believe to be the reconnection site. Shimojo et al. (1996)
reported from the statistical study of 100 jets in SXT/Yohkoh
observations that most of them were associated with micro-flares in the
foot-points of the jets. Shimojo
et al. (2007) resolved the fine structure of a quiet
Sun X-ray jet to be the expansion and eruption of loop structures
colliding with the ambient magnetic field, similar to the CH jet.
Madjarska (2010) studied in great detail one of the jet-like phenomena
identified in the datasets analysed here. The author estimated that the
reconnection site reaches temperatures of up to 12 MK from
observations of Fe XXIII 263 Å
from EIS/Hinode, which confirms that some of these events are very
similar to large flares but on a much smaller spatial scale. Seconds
after the reconnection takes place, a cloud of plasma is blown out from
the BP. Madjarska (2010) reported that although the event
appears more like a jet (although some expanding loops can also be
distinguished) in X-ray images as observed in projection on the solar
disk, the two additional view points from STEREO/SECCHI reveal that the
phenomenon evolves as an expulsion of BP loops followed by a collimated
flow along the quasi-open field lines of the expanded loops. The
escaping plasma reaches temperatures of around 2 MK (Culhane et al. 2007;
Madjarska 2010).

Nisticò
et al. (2009) found 5 out of 79 jets
analysed from STEREO/SECCHI observations exhibiting a three part
structure typically of coronal mass ejections (CMEs) - bright leading
edge, a dark void and bright trailing edge (e.g. the prominence
material). The authors named them micro-CMEs. The rest were called
Eiffel tower-type jets, where the reconnection appears to happen on top
of the loops and lambda-type jets with reconnection occurring in the
jet foot-points. As most of the events studied here were seen in
projection on the solar disk, this visual division into different
groups is not possible. However, the visual examination of the
phenomena analysed here confirms that expulsion of BP loops describes
these features best, hence, we will further refer to them as EBPLs
(Eruptive BP loops).

Figure 5:

An example of a typical jet-like happening on 2007 November 12 at the
coronal hole boundaries. A refers to the jet while B refers to
the BP.

Figure 5
presents an example of a typical EBPL event from a BP happening along
an ECH boundary. The EBPL ends with the BP vanishing at X-ray
temperatures triggering the coronal hole to expand. The plasma ejected
from the BP seems to collide with structures on its way (propagating
towards the QS region seen in projection on the solar disk) setting off
a brightening in a pre-existing BP (denoted with B in Fig. 5) with no obvious
plasma outflows.

We also found that the presence of a transient CH in the quiet
Sun triggers the occurrence of EBPL-like phenomena, similar to the
equatorial and polar CH ones. Yet again, the evolution of the BPs along
the boundaries changed the CHBs (Fig. 6). Due to the smaller
size of the TCH, these changes lead to a large expansion or contraction
of the TCH, in some cases even a disappearance. In Fig. 7 we give a series of
images which exhibit one of the quiet Sun events. In comparison to CHs,
the events identified in the quiet Sun images rarely show outflows
(Table 2).
Neither expanding loops nor collimated flows were distinguishable in
the XRT images (i.e. no EBPL).

This brings us to the numerical comparison of CHBRs and CHs
with QS areas. We find an average of 75 brightening events per
arcsec2 per day
for the coronal holes and boundaries and 20 events in the
quiet Sun. Approximately 70
of these brightenings in CHs and CHBRs showed plasma outflows while
only 30
of the brightenings seen in the quiet Sun exhibit jet-like structures.
Previous works indicated fewer events per day either because of the
poorer spatial resolution of the instrument used or because of the
visual identification methods.

The identified brightening events with no plasma outflows
could either be driven by two sided loop reconnection (Shibata et al. 1994)
between emerging fluxes and overlying coronal fields, in which the
ejected plasma flow along the closed loop structures, or reconnection
driven brightenings with plasma outflows at much lower temperatures.
They can also represent flows in loop structures, perhaps triggered by
reconnection shocks from the neighbourhood as seen in the example
brightening event B in Fig. 5. Shimojo et al. (2007)
showed that a QS jet appears to be guided by the closed magnetic field
lines (loops), unlike the jets in the CHs and CHBRs which are guided by
the open magnetic field lines. This result immediately raises the
question whether the presence of open magnetic field lines is crucial
for the generation of outflow phenomena.

Comparison of the physical properties such as duration and
size of the jet-like events in ECHs and polar CHs show no difference. A
good correspondence was found between the duration and the size of the
events, irrespective of their position. The larger events have longer
duration of around 40 min and are mostly associated with pre-existing
coronal bright points or at least with features becoming visible in
X-rays just before the eruption. The smaller events have a shorter
duration of around 20 min (mostly with no pre-existing features at
coronal temperatures).
However, for most events the actual duration from the moment the
reconnection occurs (i.e. when the plasma is ejected) until the plasma
outflow is no longer evident is usually between 10 and 15 min.
The amount of plasma ejected entirely depends on the magnetic energy
available before the reconnection and it will, therefore, differ from
event to event. Repetitive occurrence of jets in the same bright points
are more common in CHs than in the QS. No periodicity was found,
although a large number of BPs produced several jet-like events (from 2
to 5 times) during the course of the observations until all the stored
magnetic energy was exhausted and the bright point fully disappeared.

The proper motions of the outflows obtained from space-time
plots are in the range of 100-500 km s-1
for most of the events. Because of the projection effect of the jets
with respect to the solar disk, the velocity we obtain gives the lower
boundary of the real velocity of the ejected plasma. Based on their
plasma velocity we divide X-ray jets into two groups: (i) jets with
pre-existing coronal structures (X-ray BPs) which have velocities 350 km s-1
or greater (i.e. in the range of the Alfvén velocity in the lower
corona) and (ii) jets with no pre-existing structures at X-ray
temperatures, showing velocities of around 150 km s-1
(i.e. close to the Alfvén velocity in the transition region). Stereo
EUV images taken with the 171 Å filter confirm the presence of
a corresponding reconnection jet-like structures at transition region
temperatures.

5 Conclusions

The present article confirms our findings from Paper I that
the evolution of loop structures known as coronal bright points is
associated with the small-scale changes of CHBs. We were able to
identify the true nature of these changes which represent plasma
outflows associated with the expansion of bright point loop structures.
The plasma trapped in the loop structures is consequently released
along the ``quasi'' open magnetic field lines. These ejections appear
to be triggered by magnetic reconnection, most probably the so-called
interchange reconnection (Wang
& Sheeley 2004) between the closed magnetic field
lines (BPs) and the open magnetic fields of coronal holes. The ejected
plasma is guided and accelerated further away from the Sun by the open
magnetic field lines with some jets reaching several solar radii (Nisticò et al. 2009).
Contrary, in the quiet Sun the plasma ejected as a result of two or one
sided loop reconnection, is contained in the corona by the closed
magnetic field lines.

Tall and extended coronal loops are very rare in coronal holes
(Wiegelmann &
Solanki 2004), while closed (loop) magnetic structures of
varying physical properties are ubiquitous in the quiet Sun corona as
seen in EUV and X-ray observations. Coronal holes with predominant open
magnetic fields and minority closed loop structures (BPs) are
encompassed by these loop structures seen at transition region and
coronal temperatures. Woo
et al. (2004), Widing
& Feldman (2001) and Feldman et al. (2005)
showed that the elemental abundance of trapped plasma is proportional
to the confinement time of the plasma in loop structures. Newly emerged
active region loop structures were found to have initial photospheric
abundances (FIP bias 1-2)
which increased with time, reaching 5 in 1-3 days (Widing & Feldman 2001).
Schrijver et al.
(1998) concluded that 1.5 days is the
reconfiguration time-scale for the super-granulation network magnetic
fields where coronal BPs have their foot-points rooted. This time
period is in the range of the confinement time-scale needed for the
enhancement of the FIP bias.

Coronal BPs electron densities were derived from CDS onboard
SoHO in the temperature range
K
by Ugarte-Urra
et al. (2005). The authors concluded that the bright
points plasma have properties which are more similar to active region
plasma rather than quiet Sun plasma, although BPs do not show the
increase of electron density at temperature over
K,
observed in the core of active regions (Ugarte-Urra et al. 2005).
These results were later confirmed from data taken with the
Extreme-ultraviolet Imaging Spectrometer (EIS) for
and 6.2 K (Pérez-Suárez
et al. 2008). The BP lifetime in EUV were
found to be on average 20 h in the EUV (Zhang et al. 2001)
and on average 8 h in X-rays with some BPs lasting up
to 40 h (Golub
et al. 1974). The results of individually studied
BPs by Ugarte-Urra
et al. (2004) give for two BPs a lifetime of 38 and
51 h detected in Fe XII 195 Å
images from Extreme-ultraviolet Imaging Telescope (EIT) on-board SoHO
and by Pérez-Suarez (Ph.D. thesis 2009) in five BPs:
BP1 - 48 h, BP2 more than 54 h,
BP3 - 37 h,
BP4 - 45.2 h and
BP5 - 35 h (on the limb). The study by Golub et al. (1974)
on BPs lifetime in X-rays has not been updated so far using Hinode
X-ray observations. The BPs properties given above strongly suggest
that their plasma can become enriched on low FIP elements.

Raouafi
et al. (2008) concluded that X-ray jets are the
precursors of polar plumes, and jets happening in pre-existing polar
plumes enhance the brightness of the plume haze. Polar plumes are
observed even at several solar radii (Deforest
et al. 1997) and were found to contribute to the
solar wind stream. They have been reported to occur even at low
latitudes (Wang &
Sheeley 1995). Jets, associated with BPs, were also recently
registered with the Large Angle and Spectrometric Coronagraph on-board
SoHO (Wang &
Muglach 2008) and SECCHI/STEREO (Nisticò et al. 2009).
Hence, the BP plasma cloud, which is ejected as a result of magnetic
reconnection, will therefore, escape from the Sun having the plasma
characteristics of the slow solar wind. We asked ourselves whether the
plasma ejections we observe can possibly be a source of the fast solar
wind? This possibility cannot be fully rejected, although it is a fact
that these jets happen sporadically rather than continuously, which is
in contradiction with the nature of the fast solar wind.

Our specially designed observing programs provided us with
spectroscopic co-observations from SUMER, CDS and EIS along with the
XRT and SOT. In a follow up paper we will derive the physical
properties such as velocity, density, temperature and others of a large
number of events happening in the FOV of the spectrometers.

Acknowledgements

The authors thank ISSI, Bern for the support of the
team ``Small-scale transient phenomena and their contribution to
coronal heating''. Research at Armagh Observatory is grant-aided by the
N. Ireland Department of Culture, Arts and Leisure. We also
thank STFC for support via grants ST/F001843/1 and PP/E002242/1. Hinode
is a Japanese mission developed and launched by ISAS/JAXA, with NAOJ as
domestic partner and NASA and STFC (UK) as international partners. It
is operated by these agencies in co-operation with ESA and NSC
(Norway). The STEREO/ SECCHI data used here are produced by an
international consortium of the Naval Research Laboratory (USA),
Lockheed Martin Solar and Astrophysics Lab (USA), NASA Goddard Space
Flight Center (USA), Rutherford Appleton Laboratory (UK), University of
Birmingham (UK), Max-Planck-Institut für Sonnensystemforschung
(Germany), Centre Spatiale
de Liège (Belgium), Institut d'Optique Théorique et Appliquée (France),
and Institute Astrophysique Spatiale (France).

Footnotes

All Tables

Table 2:
Number of events over 24 h per 100
100 arcsec2 identified inside the coronal holes
(CH), in coronal hole boundary regions (CHBR) and in the quiet Sun (QS).

All Figures

Figure 1:

Equatorial coronal hole ( top left), polar CH (
top right), quiet Sun ( bottom left) and
quiet Sun with TCHs ( bottom right) with the
positions of all the corresponding identified brightening pixels
over-plotted. The CH boundaries are outlined with a black line. The
over-drawn rectangles correspond to the field-of-views shown in
Figs. 5-7. Time sequences can
be found in movies online.

Equatorial coronal hole observed by XRT on 2007 November 9, 14 and 16
with the positions of all the identified brightening pixels
over-plotted. The CH boundaries are over-plotted with a black solid
line.

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